Distance-based detection in analytical flow devices: From gas detection tubes to microfluidic chips and microfluidic paper-based analytical devices

Distance-based detection (DbD) in analytical flow devices was first described in the 1930s, based on a colorimetric measurement of gas samples passing through a tube with a solid porous packing showing a color change as it reacts with the gas, with the zone length proportional to the analyte concentration. Over the following decades, DbD was introduced to a variety of formats and platforms including capillary, microfluidic chips and microfluidic paper-based analytical devices. Most of the materials are easily functionalised, and disposable. The DbD principles are based on a visible color change resulting from various reactions ranging from simple colorimetric to specific ligand or antibody binding. The immense attractivity of DbD rests in its simplicity of quantitative readout without auxiliary instrumentation. This review outlines key historical developments during 1937–2021, detection principles, driving forces, instrumental formats, and application areas. Finally, ways to overcome limitations, improve the performance and future perspectives are discussed.